As a method of removing a resist from a wafer surface in a semiconductor manufacturing process, the SPM method (acronym of Sulfuric acid and hydrogen Peroxide Mixture: cleaning method using a mixture of concentrated sulfuric acid and hydrogen peroxide) has conventionally been used widely. However, in the SPM method, hydrogen peroxide is mixed with the cleaning liquid (concentrated sulfuric acid solution) each time when the cleaning liquid is used, and therefore the concentration of the sulfuric acid decreases along with mixing with the hydrogen peroxide. After a certain number of times of the mixing, the oxidization power of the mixture decreases. On such an occasion, the cleaning liquid is to be disposed of, and an increased chemical cost and an environment load pose a problem.
In order to address this problem, a persulfuric acid method compensating the defects of the SPM method is proposed (refer to a patent document 1). The persulfuric acid method generates the persulfuric acid (peroxodisulfuric acid) by electrolyzing a sulfuric acid solution, and a strong oxidization power of the persulfuric acid is used. The persulfuric acid oxidizes and decomposes the resist into CO2 and H2O, and then returns to the sulfuric acid, and recycling is thus realized by returning this liquid to an electrolysis cell. As a result, a higher merit over the SPM method is thus expected in terms of the reduction in the chemical cost and the environment load.
An electrode used for the electrolysis of sulfuric acid is referred to as “diamond electrode”, and an electrode manufactured by depositing diamond crystals on a silicon substrate to form a diamond layer in an order of some micrometers to some tens of micrometers, and by implanting (doping) boron thereto for conductivity is usually used. This electrode is used as an anode and a cathode of an electrolysis cell. It is considered that the employment of the diamond electrodes eliminates degradation of the electrodes even at a high current density, resulting in a stable electrolysis process.    Patent document 1: Japanese Patent Application Laid-Open No. 2006-114880